The present invention relates to a control apparatus for an automatic transmission which is capable of performing a shock-less gear shifting operation when a target gear position is suddenly replaced or renewed with another gear position in response to a new gear shift request (i.e. a requested replacement of target gear position according to driver's preference or driving conditions of a vehicle) which may be raised during a shifting operation of gear position in a speed change mechanism.
According to an automatic transmission for an automotive vehicle, the engine power is transmitted via a torque converter to an input shaft of a speed change mechanism. The speed change mechanism, having the capability of selecting a changing gear ratio, outputs a rotational force from its output shaft and transmits a changed rotational speed to respective driving wheels. A generally known speed change mechanism includes a plurality of gear trains disposed between an input shaft and an output shaft to arrange a plurality of power transmission routes between the input and output shafts which are differentiated in the change gear ratio. The speed change mechanism includes frictional engaging elements, such as clutches and brakes, which are provided for selectively establishing each power transmission route. The fluid pressure applied to respective frictional engaging elements is separately controlled in accordance with a gear shift request. By performing a charge control for charging hydraulic fluid to a predetermined frictional engaging element and a drain control for discharging hydraulic fluid from this frictional engaging element, engagement/disengagement of this frictional engaging element is switched to complete the shifting operation of gear position in the speed change mechanism. In this manner, the power transmission route is selectively established between the input and output shaft of the speed change mechanism so as to switch the change gear ratio.
According to this kind of automatic transmission, a new gear shift request requiring replacement or renewal of target gear position may arise during a currently implementing shifting operation of gear position. In such a case, the speed change mechanism performs a multistage gear shifting operation in response to the new gear shift request. A multi-stage gear shifting control for the speed change mechanism includes interruption of the currently implementing shifting operation to the initial target gear position and reset of target gear position for finally shifting to a replaced or renewed target gear position. The multi-stage gear shifting control forcibly interrupts a charge control for charging hydraulic fluid into a cylinder of a certain clutch or brake, and immediately starts a drain control for discharging the hydraulic fluid from this cylinder in accordance with replacement or renewal of target gear position. Furthermore, if the request to replace or renew the target gear position again arises during the drain control having been just started, it will be necessary to immediately start the charge control again. However, at the moment of such a quick switching of charge/drain control, the state of hydraulic fluid draining from the cylinder may be incomplete. There will be a possibility that excessive hydraulic fluid may rush into the cylinder when the charge control is performed and a significant amount of shift shock (i.e. gear engagement shock) will arise.
To suppress the shift shock occurring during the multistage gear shifting control, Japanese Patent No. 3291970 discloses a technique for completing a shifting operation to a first gear position before starting a shifting operation to a second gear position when a switch request requiring the shifting operation to the second gear position arises during the shifting operation to the first gear position.
However, according to the above-described conventional technique, it will take a long time to complete all of the shifting operations to the first and second gear positions which are sequentially performed.
In this respect, the Japanese Patent No. 3301344 discloses an improved technique, which proposes detecting a fluid pressure supplied to a frictional engaging element actuated to establish a first gear position when switching to a second gear position is requested during a shifting operation for the first gear position. The detected fluid pressure value is usable as a factor indicating the state of currently progressing shifting operation to the first gear position. When the judgment result shows no possibility of causing any substantial shift shock during a multistage gear shifting control, the shifting operation to the second gear position immediately starts in response to the switch request. On the other hand, in a case that the judgment result predicts a significant shift shock, the shifting operation to the second gear position is delayed until the shifting operation to the first gear position is completed.
However, according to the above-described gear shifting control disclosed in the Japanese Patent No. 3301344, when the switch request to the second gear position arises during the shifting operation to the first gear position, only the state of currently progressing shifting operation for the initial gear position (i.e. first gear position) is relied in making a judgment as to whether any shift shock will occur during the multistage gear shifting control. In other words, no consideration is given to the charged/discharged condition of a frictional engaging element to be newly charged to establish the replaced or renewed gear position (i.e. second gear position) in response to the switch request. Thus, it was difficult to accurately predict the shift shock actually occurring during the multistage gear shifting control. More specifically, it depends on the state of hydraulic fluid draining from a frictional engaging element to be newly charged to establish the replaced or renewed gear position (i.e. second gear position) if any shift shock will occur when hydraulic fluid rushes into this frictional engaging element. Thus, a significant shift shock will occur due to excessively supplied hydraulic fluid when the charge control is started for this frictional engaging element. In short, there is no correlation between the state of currently progressing charging operation for one frictional engaging element to establish the initial gear position and the state of hydraulic fluid draining from another frictional engaging element to be newly charged to establish the replaced or renewed gear position. Only relying on the former in accurately predicting the shift shock is not effective at all, because the latter is another key factor possibly causing the shift shock. Thus, according to the above-described conventional gear shifting control, it is impossible to prevent the shift shock from occurring during the multi-stage gear shifting control.